The principal objective of this project is to measure the distribution of Na ions- K ions ATPase dependent transport sites in the vertebrate retina. The enzyme of intact isolated frog, turtle and cat retinas is tagged with 3H-ouabain and its location ascertained by quantitative high resolution autoradiographic techniques for water soluble compounds (J. Cell Biol 35:585 and 53:704). In pursuance of this goal, the kinetics of ouabain binding are measured in order to determine the maximum binding capacity of each retinal layer. Also, the extracellular volume of each layer is measured autoradiographically with 3H-mannitol. These data together with estimates of the spacing between adjacent cell membranes will be used to calculate the average membrane transport site density of each layer. Another aim is to examine the relative permeability of each layer to small organic molecules by measuring the kinetics of 3H-mannitol and 3H-ouabain penetration into the retina. The binding measurements will provide direct experimental evidence regarding the nature, location and capacity of the cation pump which maintains the dark current of photoreceptors. Also, they will provide a better understanding of the ionic pumping loads of the various cell types, including their processes and may provide a clue to the existence of local ionic currents in addition to that of the photoreceptor layer. The retinal penetration measurements will reveal any nonlinearities in the extra-cellular diffusional resistance of the retina to small water soluble molecules.